I think it's pretty clear now, the tale of the Lover's Abyss is repeating.
I thought Nagi would fit the beautiful girl who came from the moutains and was desired by everyone, even Gen kinda implies it, that there would be jealousy if word got out that a beautifull girl like her was dating a guy (Reigi).
BUT INSTEAD it is Reigi who's taking that role. He is desired not only by the teacher but now by his own childhood friend, and both of them are starting to show symptoms of possessiveness and probably later jealousy.